LaCoste
& Romberg LLC
The first name in gravity
since 1939
MVR FEEDBACK
SYSTEM MANUAL
CHAPTER 2
OPERATION
To use the MVR Feedback
system, first null the gravity meter in the standard manner with
the Feedback OFF. This means turning the nulling dial until the
beam is observed (in the eyepiece) to be at the reading line.
Then turn the MVR POWER ON
to the 10-Volt position, and observe the beam in the eyepiece. It
should be held at the reading line as you turn the nulling dial
approximately one dial turn (10 turns on D-Meter) either up or
down. Moving the dial further will exceed the range of the
Feedback system, and the beam will go to either the top or bottom
stop.
MVR POWER/RANGE SWITCH
The range of the MVR Feedback card depends on the DC
voltage supplied to the card. A switch on the black lid allows
the user to select between OFF, 10 and 30 volts DC for the MVR
card power supply.
With the Feedback power
supply set to 10 volts, a feedback range of about 2 milligals is
typical. This is the most suitable for tide monitoring and
microgravity surveying.
With the Feedback power
supply set to 30 volts, a feedback range of about 20-25 milligals
is available. This is typically used for point-to-point field
surveying where larger changes in observed gravity are expected.
When the switch is OFF, the
MVR card has absolutely no effect upon the gravity meter. You can
read the meter optically, and move the beam to the reading line
using the nulling dial, as you would when taking a normal gravity
reading with any L&R land gravity meter.
READING LINE ADJUSTMENT
The Reading Line
adjustment potentiometer is labelled "MVR ZERO". It is
accessible through a small hole on the black lid of the gravity
meter.
The pot should be adjusted so that when the Feedback card is
ON, it forces the beam to the Reading Line specified on the black
lid. First, null the meter optically with MVR POWER OFF, and then
turn MVR POWER ON to make this adjustment.
The gravity meter operates
from 12 VDC. This DC power is normally supplied from a battery or
AC-to-DC power supply. The 10 or 30 volts supplied to the MVR
Feedback card is derived from a DC-to-DC Converter and precision
voltage regulators connected to the system 12 VDC power.
However, no voltage
regulator is perfect, so the stability of the external voltage
supply is important. Large changes in the voltage supplied could
cause changes in the output voltage of the MVR Feedback system.
Use the Battery Voltage position on the System Monitor on the
black lid to observe the change in supply voltage as the meter
heating circuitry goes on and off. A change of a few tenths of a
volt are usually acceptable, but a change of 1 or 2 volts
indicates the power supply is not adequate.
When observing the beam in
the eyepiece, a small oscillation of the beam around the reading
line may be observed. This is normal, and is the result of
hysteresis between the positions where the forcing signals
reverse as the mass goes up or down.
A nulling dial clamp is
recommended for some applications, in particular microgravity
surveying and tide monitoring. The clamp will ensure that the
dial is not moved during survey or tide monitoring.
When operating from a pure
floating power supply such as a battery, it may not be necessary
to ground the gravity meter. However, if the power supply is from
an AC source, a battery charger is connected to the battery, or a
Data Acquisition system is attached, it can be critical to ground
the meter for usable results.
The standard analog output
connector on the side of the gravity meter is a three-conductor
"stereo" type miniature plug. The two inner conductors
are the floating differential MVR voltage output. The outer
conductor is a ground connection directly to the gravity meter
aluminum case.
The analog output cable has
a three-connection pin on one end, and two banana plugs (for
Fluke DVM) and alligator clip (for earth ground) on the other
end.
Table of
Contents
Chapter
3
Product Information
Copyright
© 1998 LaCoste and Romberg LLC